This invention relates to valve train assemblies, and particularly but not exclusively to assemblies which incorporate a rocker arm pivoted by a cam in order to operate a valve.
A typical structure of this type is shown schematically in FIG. 1. The valve train assembly 2 comprises the rocker arm 4 and a hydraulic lash adjuster 6. One end 8 of the rocker arm 4 engages the stem 10 of a valve 11. The other end 12 of the rocker arm is mounted for pivotal movement on the lash adjuster 6.
The rocker arm 4 is provided with a roller 14 mounted on an axle 16 carried by the rocker arm 4.
A cam 18 mounted on a cam shaft 15 has a lobe 17 which can engage the roller 14 and thus pivot the rocker arm 4 counterclockwise as shown in the drawing. This depresses the valve stem 10 against the force of a valve spring (not shown) and thus opens the valve. As the cam continues to rotate, and the base circle 19 of the cam profile again engages the roller 14, the valve spring returns the valve and the rocker arm 4 to the position shown in FIG. 1.
As is well known, a hydraulic lash adjuster has an oil-containing chamber and a spring arranged to enlarge the chamber and thus extend the lash adjuster. Oil flows into the chamber via a one-way valve, but can escape the chamber only slowly, for example via closely-spaced leakdown surfaces.
Accordingly, the lash adjuster 6 of FIG. 1 can extend to accommodate any slack in the valve train assembly, such as between the cam 18 and the roller 14. After it is extended, however, the oil-filled chamber provides sufficient support for the pivoting movement of the rocker arm 4.
It is important for the base circle 19 of the cam 18 to be concentric with respect to the axis of rotation of the cam shaft 15. Any slight eccentricity (xe2x80x9crun-outxe2x80x9d) could cause the valve to close later than it should, or open during the movement of the base circle past the roller 14. The cam 18 is often formed by sintering and may not have, in its initial state, particularly accurate dimensions. Accordingly, it is conventional, before assembly, to grind either the outer surface, including the base circle 19, of the cam 18, or to grind the inner diameter which is fitted to the cam shaft 15, to ensure accurate concentricity of the base circle 19 relative to the axis of rotation of the cam shaft 15.
Although the arrangement described above works well during normal running conditions, problems can arise in certain circumstances. For example, in order to prevent problems when starting the engine from cold, it has been proposed to use a technique whereby the valves and cylinder head are caused to heat up very quickly. Referring to FIG. 2, the rapid heating of the head 23 of the valve 11 causes the head 23 to expand relative to the valve seat 21. This expansion results in the valve moving downwardly against the force of the valve spring, as shown on the right of FIG. 2. This process creates positive lash, which is accommodated by expansion of the hydraulic adjuster as the camshaft rotates. However, as the cylinder head 25 and the valve seat 21 then heat up, their expansion allows the valve 11 to move back upwardly, thus creating negative lash (which will be subsequently exacerbated due to expansion of the valve stem 10). This negative lash can be accommodated by shrinking of the lash adjuster. However, because the heating process is taking place rapidly, and the shortening of the lash adjuster is limited by the rate of leakage of oil from the high pressure chamber, the lash adjuster does not shorten sufficiently quickly. This problem is exacerbated because the oil is still cold and therefore viscous, thus reducing the leakage rate. This results in valves remaining open (shown in dotted lines in FIG. 1), causing starting problems.
There have been proposed valve trains which provide xe2x80x9clift lossxe2x80x9d, that is, in which the initial movement caused by the cam is absorbed before further movement is transmitted to the valve. See for example U.S. Pat. Nos. 6,039,017 and 6,170,450. Thus, there is a degree of lost motion in the valve train before the valve starts to open. This lost motion is recovered by a spring after the valve has closed. Using such an arrangement, a small degree of negative lash can be quickly accommodated by the lost motion, thus making it more certain that the valve will close.
It would be desirable to provide an improved arrangement for providing lift loss, and also to provide a valve train which can be manufactured more easily.
Aspects of the present invention are set out in the accompanying claims.
According to one aspect of the invention, a valve train assembly includes a rocker arm supporting a cam-engaging roller, wherein the roller, instead of having a rotation axis fixed with respect to the rocker arm, has an axis which is allowed to shift slightly against the force of a resilient biasing means, thereby providing lift loss when operating the valve, and thus accommodating movement of the valve towards its closing position in circumstances in which a lash adjuster has extended excessively.
In the preferred embodiment, the resilient biasing means, or spring, is strong enough to prevent the lash adjuster from taking up the lost-motion connection in the valve train. As the lobe on the cam starts to operate on the valve train, the lost-motion connection permits such movement to occur, without operating the valve, against the force of the spring. After the lost-motion has been taken up, the valve is operated. Subsequently, as the cam continues to rotate, the valve moves towards its closing position. After the valve is closed, the lost-motion connection is restored by the biasing means. Closing of the valve is thus guaranteed, because any required additional movement is accommodated by the lost-motion connection.
According to another aspect of the invention, it has been perceived that a lost motion connection such as that set out above means that the base circle radius variation of the cam no longer has to be minimized by grinding, allowing the use of net-shaped cam shaft technology instead of more expensive ground cams. (The term net-shaped is generally understood, and used herein, in the sense of having a shape and dimensions which are at least substantially the same as those resulting from the initial forming of the object. This does not exclude the possibility of small changes in dimensions which are a consequence of, for example, surface-treatment for the purpose of smoothing, as distinct from dimensional changes (e.g. by grinding) for the purpose of altering the function performed as a result of those dimensions. The initial forming may, but is not necessarily, achieved by sintering.)
The invention will be described in the context of an arrangement in which the valve assembly comprises a rocker arm pivoted at one end on the lash adjuster, with the other end operating the valve stem. However, some aspects of the invention can also be embodied in other types of arrangements, such as center-pivoted rocker arms, and indeed could be applied to arrangements which don""t include a rocker arm, such as direct-acting tappets (see U.S. Pat. No. 6,170,450), so long as the requisite lost motion connection is provided.